Optimized is that:
[MyFirstModule.Nachricht_Freund/MyFirstModule.Freund/MyFirstModule.Freund_Personen/MyFirstModule.Personen
[vorname = 'Anna' and nachname = 'Kraus']]
Here you can find the whole xml file - http://www.file-upload.net/download-11665751/social.xml.html
I recreated your schema. Let's assume you are linking many Nachricht objects to one Fruend Object, and many Fruend Objects to one Personen object. In English, A person can have multiple emails, and each email account can be related to multiple Emails. I would tackle your queries as such:
Something like this in Xpath, or break it up if necessary (didn't have time to build data and test). Retrieve from the Personen entity [MyFirstModule.Freund_Personen/MyFirstModule.Freund/MyFirstModule.Nachricht_Freund/MyFirstModule.Nachricht/ MyFirstModule.Nachricht_Freund/MyFirstModule.Freund/MyFirstModule.Freund_Personen/MyFirstModule.Personen/vorname = 'Anna'] [MyFirstModule.Freund_Personen/MyFirstModule.Freund/MyFirstModule.Nachricht_Freund/MyFirstModule.Nachricht/ MyFirstModule.Nachricht_Freund/MyFirstModule.Freund/MyFirstModule.Freund_Personen/MyFirstModule.Personen/vorname = 'Kraus']
Retrieve from the Personen Entity [MyFirstModule.Freund_Personen/MyFirstModule.Freund/MyFirstModule.Nachricht_Freund/MyFirstModule.Nachricht]
Retrieve from the Nachricht Entity [MyFirstModule.Nachricht_Freund/MyFirstModule.Freund/MyFirstModule.Freund_Personen/MyFirstModule.Personen/vorname = 'Anna'] [MyFirstModule.Nachricht_Freund/MyFirstModule.Freund/MyFirstModule.Freund_Personen/MyFirstModule.Personen/nachname = 'Kraus']